1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission.
2. Description of Related Art
Conventionally, an automatic transmission has a gear box including a gear train and is constructed so as to input rotation at a first speed to a certain gear element in the gear train and to output rotation at a second speed from another gear element. Toward that end, a plurality of frictional engagement elements, such as clutches and brakes, are provided to selectively transmit the rotation between different combinations of gear elements by selectively engaging and disengaging the frictional engagement elements to provide a plurality of gear speed stages, each having a given operative combination of the gear elements.
In the case of a hydraulically controlled automatic transmission, each of the frictional engagement elements has a corresponding hydraulic servo. A gear speed change is effected by supplying oil to a hydraulic servo for a disengaged frictional engagement element to engage same and draining oil from a hydraulic servo for an engaged frictional engagement element to disengage same.
However, if oil is abruptly supplied to a hydraulic servo, its associated frictional engagement element will abruptly engage, thereby causing a gear change shock. Accordingly, gear change shock is reduced by supplying oil to the hydraulic servo with an appropriate transient characteristic.
Gear change shock caused by an excessive or deficient operating pressure may be also reduced, and life of the frictional engagement elements prolonged, by governing a line pressure in accordance with input torque to the transmission. In one such prior art control system, an input torque is estimated, based on a fuel injection pulse and engine speed, and a line pressure is governed in accordance with the estimated input torque to adjust operating pressure of a hydraulic servo to an adequate value (hereinafter referred to as "the first prior art automatic controller". See Japanese Patent Laid-Open no. Hei 1-116363.)
Another prior art control system governs a line pressure so that the elapsed time for a gear change agrees with a targeted value, which targeted value depends upon the frictional engagement elements involved in the gear change, deterioration of the oil and variables in manufacture (hereinafter referred to as "the second prior art controller". See Japanese Patent Laid-Open no. Hei 2-31069.)
Still another prior art automatic transmission controller has been provided which sets a targeted rotational speed locus (graph) for a rotary member involved in a gear change, which targeted locus is determined in accordance with the frictional engagement elements involved in the gear change, state of deterioration of oil and other variables in production, to control the operating pressure of a hydraulic servo so that the speed of revolution of the rotary member changes along the locus. (Hereinafter referred to as "the third prior art controller". See Japanese Patent Laid-Open no. 63-212137.)
In controllers of the prior art automatic transmissions described above, however, the gear change is slowed down if the gear change is made while traveling at high-speed and a large gear change shock results if the gear change is made while traveling in low-speed in the case of the first prior art controller.
Thus, while the first prior art controller governs the line pressure in accordance with the input torque to make the gear change by engaging the appropriate frictional engagement elements, the frictional engagement elements receive an inertia torque for changing the speed of revolution of the rotary members (hereinafter referred to as the "inertia phase") when the speed of revolution of the rotary members on the input side, such as the engine and torque converter, is changing. Therefore, the hydraulic servo is required to operate with higher operating pressure to account for the inertia torque which the frictional engagement elements receive.
The inertia torque can be generally represented by a value obtained by multiplying the moment of inertia of the rotary members by angular acceleration (change of speed of revolution) of the rotary members. Accordingly, even if the gear change is made with the same input torque, a larger inertia torque is generated when the speed of revolution of the input side rotary members is high and a smaller inertia torque is generated when the speed of revolution of the input side rotary members is low. As a result, the frictional engagement elements cannot receive a sufficiently large inertia torque while at high-speed, making the gear change slow, and do receive full inertia torque while traveling in low-speed, causing a large gear change shock.
In contrast, because the second and third prior art controllers have operating pressure of the hydraulic servo controlled to correspond to gear change time and speed of revolution of the rotary members in the inertia phase, a control gain has to be increased in order to have the operating pressure correspond to the large change in torque caused when the frictional engagement elements receive the inertia torque. However, the increase of the control gain prevents the operating pressure from being finely adjusted and from fully accommodating for deterioration of the frictional engagement elements and oil and variations in manufacturing.
Accordingly, it is an object of the present invention to solve the aforementioned problems of the prior art controllers by providing a controller for an automatic transmission which allows a gear change to be made favorably under any traveling conditions and to always provide the best gear change characteristics.